Fixed uses of get() to unwrap() in doc examples in std::iter
Fixed docs for advance() in Iterator trait
auto merge of #10533 : tautologico/rust/fixdocs, r=huonw

This fixes a number of bugs in the doc comments for a bunch of functions in libstd/iter.rs, mostly updating to use unwrap() instead of get() on options. Also fixes the docs for advance() (trait Iterator) which was not making sense, though if it is not useful anymore maybe it should be removed.